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North Korea has executed at least five senior security officials with an anti-aircraft machine gun after their boss was detained for making an unspecified false report, South Korea's spy agency said Monday.

The National Intelligence Service (NIS) said in a report to a parliamentary intelligence committee that more officials from North Korea's top state security body could be executed as a special audit by the ruling Workers Party is under way. Lawmakers who attended the closed session conveyed to reporters what NIS officials said.

The South's unification ministry said earlier that Kim Won-hong, 72, was fired as head of the Ministry of State Security in mid-January with his military rank demoted from full general to major general for alleged corruption and abuse of power.

The security body has been under scrutiny by party inspectors over alleged abuse of power, corruption and human rights violations such as torture, the ministry said, suggesting that Kim Won-hong is suspected of having lost a struggle for power.

The man was among a gang of power holders trusted by the leader who has been accused of resorting to the reign of terror to consolidate his power since he took office in late 2011 in a dynastic third-generation transfer of power.

There have been sporadic reports of purges and executions involving senior party, government and military officials in North Korea. The most notorious case was reported in December 2013, when the leader executed his influential uncle Jang Song-taek on charges including treason and corruption.

Kim's repressive dictatorship has led to the defection of some North Korean elites that Seoul says points to signs of cracks in the North's regime. Experts say Kim Won-hong, who became the head of the security body in April 2012, is known to have led a political campaign against Jang and his associates.

The unification ministry said North Korean elites would become more agitated because Kim Won-hon was a key figure in supporting the leader's reign of terror. The North's security ministry has been used by North Korean leaders to prop up their power and eliminate so-called reactionary elements in society and political opponents.